THE ICECUBE NEUTRINO OBSERVATORY

1. INTRODUCTION
2. THE ICECUBE CONCEPT
3. PHYSICS PERFORMANCE
4. DRILLING AND DEPLOYMENT

O. Botner for the IceCube Collaboration*

Division of High Energy Physics, Uppsala university, Box 535, SE – 75121 Uppsala, Sweden

IceCube is a kilometer-scale deep-ice neutrino observatory under construction at the South Pole. This

paper presents a brief overview of the IceCube concept and components, its science goals, the expected

performance and the present status of the project.

1. INTRODUCTION

The prime objective for the IceCube neutrino

telescope [1] is the discovery of ultra-high

energy cosmic neutrinos considered a promising

tool for further exploration of the universe. Such

observations would be expected to shed light on

the mechanisms of acceleration of the highest

energy cosmic rays and allow a mapping of their

sources. They might also provide essential

insights regarding the origin of the observed

high-energy

rays. Neutrino production requires

hadronic processes for which two scenarios are

implied in the literature [2]: in the top-down

models neutrinos arise as decay products of

massive (M ~ 10

eV) cosmological remnants;

in the bottom-up models neutrinos originate in

decays of pions which are produced when

charged particles, accelerated to ultra-high

energies by ”cosmic accelerators” driven by

gravitational energy release, interact with dense

clouds of radiation or matter surrounding the

source. The possible ”accelerators” include

Active Galactic Nuclei (AGN), microquasars,

supernova remnants (SNR) and colliding neutron

stars or black holes (GRB). Hence, search for

steady, variable or transient sources of high-

energy neutrinos is an important science goal for

IceCube.

Apart from the astrophysics objectives, IceCube

plans to investigate topics related to particle

physics, such as signatures of dark matter or

extra dimensions, and to search for magnetic

monopoles and exotic superheavy objects like

____________________

* reference to full author list at the end

nuclearites or Q-balls [2]. In addition, owing to

the low dark noise rate of the optical modules

(OM) in ice (< 1 kHz), IceCube will be sensitive

to a coherent OM rate increase due to MeV

neutrinos from potential SN explosions within

our galaxy. IceCube will participate in the SN

alert network SNEWS [3].

2. THE ICECUBE CONCEPT

The conceptual design of IceCube is shown in

figure 1. IceCube is an array of 80 strings – in

total 4800 OMs – deployed in a hexagonal

pattern over an area of ~ 1 km

Figure1. IceCube and IceTop

The OMs are connected to the strings at depths

between 1400 and 2400 m where the optical

properties of the ice are well understood owing

to measurements with the AMANDA set-up [4].

The distance between strings, 125 m, and the

distance between OM’s in a string of about 17 m,

is a result of an optimization for the principal

energy range (TeV- PeV) of interest to IceCube

[5].

The deep-ice detector is complemented by an

Extensive Air Shower array on the surface –

IceTop – consisting of 2 frozen water tanks with

2 OM’s each on top of every string. In total 320

modules. The combination of IceCube with a

surface array provides a unique possibility of

tagging muons produced by cosmic rays in the

atmosphere which penetrate to the deep-ice

array. A valuable aspect is that the IceCube OMs

can be surveyed and calibrated

in situ

. Also

important is the possibility of vetoing some

atmosphere. All events with E > 300 TeV can be

vetoed when the shower passes the surface array.

The geometrical reach of the veto increases with

IceTop/IceCube may be used for cosmic ray

studies. IceTop has full efficiency in an energy

range < 10

– 10

eV. This includes the “knee”

of the cosmic-ray spectrum around 3.10

eV as

well as the likely beginning of the transition from

galactic to extra-galactic cosmic rays above

eV.

The fundamental component of IceCube is the

Digital Optical Module, DOM, figure 2. Each

DOM is a self-contained “mini” data acquisition

system (DAQ) capable of recording signals from

an optical sensor, digitizing and storing these

with minimal loss of information and

transmitting them to the surface at request. The

optical sensor is a 10 inch diameter, 10 stage

Hamamatsu R-7081 PMT coupled via an UV

transparent optical gel to the glass pressure

housing enclosing both the sensor and the

electronics. The housing is designed to withstand

a pressure of at least 600 bar.

The DOM digitizes the PMT waveform ensuring

a dynamic range of 200 photoelectrons (p.e.)

within the first 15 ns, a total of 2000 p.e. within 5

µs

and a dead time < 1%. Each waveform is

time-stamped locally, allowing to achieve a

relative timing accuracy between modules of

~ 7 ns, making this contribution to the

reconstruction efficiency for neutrinos small

relative to the effects of light scattering. The

Figure 2.

Schematics of a Digital Optical

Module showing the PMT, the three electronics

boards and the mu-metal shield enclosed in the

pressure sphere.

basic performance of the IceCube design has

been verified with the AMANDA string 18

deployed at the South Pole in January 2000,

comprising 40 prototype digital optical modules

connected both to the standard AMANDA DAQ

and to a prototype digital DAQ. Waveforms have

been recorded and analyzed, and down going

muons detected. Also, the critical aspects of time

stamping have been verified.

3. PHYSICS PERFORMANCE

IceCube will be sensitive to all neutrino flavors

over a large energy range. Muon tracks from

can be reconstructed above 10

eV. Cascades

from

can be reconstructed from 10

and v

can be identified above 10

eV. IceCube

performance for muons at TeV – PeV energies

has been investigated in detail in [6] whereas its

performance at EeV energies has been studied in

[7].

Figure 3 (top) shows the effective area for

neutrino-induced muons as a function of the

zenith angle for 4 different muon energy ranges.

A generic E

–2

spectrum has been assumed and

cuts have been applied to ensure track quality

and suppress the down going atmospheric muon

background by a factor of 10

. An effective area

of 1 km

will be achieved for upward-moving

muons in the tens of TeV range. The sensitive

aperture of the array increases with increasing

energy and in the PeV range full sky

observations become possible.

Figure 3 (bottom) shows the pointing resolution

(median space angle error) for muons originating

from

‘s as a function of the zenith angle, after

application of background suppression cuts.

Figure 3. Effective area for muons originating

from neutrinos as a function of zenith angle

(top). Pointing resolution (median space angle

error) for neutrino-induced muons as a function

of zenith angle (bottom).

approaching 0.6º at the horizon for muons with

energies of the order tens of TeV.

Figure 4 shows the sensitivity of IceCube for

diffuse fluxes of astrophysical neutrinos.

Assuming a generic diffuse E

–2

spectrum and

optimizing the cuts for best sensitivity we obtain

a 90% C.L. exclusion limit of 4.2 . 10

-9

–2

–1

GeV (horizontal full line) for 3 years of data

taking. The second full line shows the

corresponding limit for a generic E

–1

spectrum. The limits have been evaluated

assuming the recombination quark-parton model

for the charm contribution [8]. Other models for

charm [9] would result in improved limits. The

dashed (dotted) line in figure 4 indicates the

expected diffuse

flux from a model of photo-

hadronic interaction in AGN cores (jets) [10, 11].

The model rejection factors (

mrf

) indicate a

sensitivity of

mrf

at 90% C.L. for a model

predicting a flux

after 3 years of data taking.

Figure 4. Expected sensitivity of IceCube (3

years of data taking) to a diffuse flux of

neutrinos assuming a E

–2

spectrum.

A considerable improvement of the overall

sensitivity for diffuse sources is expected when

exploiting IceCube capability to detect cascades

from

. An effective volume of ~ 1 km

will be achieved for cascades above 10 TeV.

Possible powerful sources of high-energy

neutrinos like AGN’s will be searched for by

looking for an excess of events from within a

limited region of the sky. An expected angular

resolution in the sub-degree range will allow

IceCube to restrict the search window, thereby

reducing the background, while loosening other

cuts to retain a larger fraction of the signal.

Optimizing the cuts for best sensitivity we obtain

a 90% C.L. exclusion limit of 2.4 . 10

-9

–2

–1

GeV for 3 years of data taking.

Figure 4 also shows (dash-dotted) the prediction

from the GRB model by [12]. Assuming

independently available spatial and temporal

information, IceCube expects after 1 year of data

taking to achieve a sensitivity at the level of 20%

of the flux predicted by this model.

4. DRILLING AND DEPLOYMENT

The IceCube strings will be deployed in 60 cm

diameter holes drilled in the South Pole glacier

utilizing water at 80

°C

. The Enhanced Hot

Water Drill (EHWD) represents a development

of the drill technology used in the case of

AMANDA, making it possible to drill a vertical

hole to a depth of 2450 m within ~ 40 hours. Up

to 18 holes per season are foreseen. Mounting,

testing and lowering of a string with 60 DOMs is

expected to take about 20 hours. All drill

components have been dispatched to the South

Pole, to be used for the first time during the

2004/2005 campaign.

Figure 5. The hose-reel for the IceCube Hot

Water Drill assembled at the South Pole, January

2004.

5. STATUS OF THE ICECUBE PROJECT

AND CONSTRUCTION.

The IceCube collaboration includes about 150

researchers from 26 institutions in Belgium,

Germany, Japan, the Netherlands, New Zealand,

Sweden, U.K., USA and Venezuela. The funding

in the US is provided by the NSF, since 2004

through an MRE grant. Significant funding has

also been granted in Germany, Belgium and

Sweden. After an initial start-up phase, it is now

foreseen to deploy the very first IceCube strings

in January 2005.

The initial IceCube strings will be positioned in a

pattern close to the AMANDA array, to provide

both the possibility of calibration of IceCube

with well-defined muon tracks and, more

importantly, to ensure continuous science output

during IceCube construction. Finally, AMANDA

will become a fully integrated low threshold sub-

array within IceCube. Construction of IceCube

should be completed in January 2010.

REFERENCES

1. http://icecube.wisc.edu

2. J.G. Learned and K. Mannheim, Annu. Rev.

Nucl. Part. Sci. 2000.50:679

3. http://cyclo.mit.edu/snnet

4. J. Ahrens et al., Phys. Rev. D66 (2002)

012005

5. M. Leuthold, Proc. Workshop on Large

Neutrino Telescopes, Zeuthen 1998

6. J. Ahrens et al., Astropart. Phys. 20 (2004)

507

7. S. Yoshida, R. Ishibashi, H. Miyamoto, Phys.

Rev. D69 (2004) 103004

8. E.V. Bugaev et al., Phys. Rev D58 (1998)

054001

9. M. Thunman, G. Ingelman, P. Gondolo,

Astropart. Phys. 5 (1996) 309

10. F.W. Stecker and M.H. Salamon, Space Sci.

Rev. 75 (1996) 341

11. K. Mannheim, R. Protheroe, J. Rachen,

Phys.Rev. D63 (2001) 023003

12. E. Waxman and J. Bahcall, Phys. Rev. D59

(1999) 023002

ICECUBE AUTHOR LIST

http:

//icecube.wisc.edu/pub_and_doc/conferences/